The invention relates to a color picture screen, in particular a plasma picture screen, provided with a blue phosphor layer.
Additive color mixing takes place for displaying color pictures on picture screens. A major portion of the colors occurring in nature can be represented through additive mixing of the three primary colors red, green and blue in suitable intensities. This principle is used both in emitting picture screens such as cathode ray tubes or plasma picture screens and in non-emitting picture screens such as liquid crystal picture screens.
The representation of the various colors is laid down in so-called standard color curves. A widely used standard is the CIE color triangle. The range of the colors which can be represented by a picture screen is defined by the color points of the three phosphors, which again are dependent on the respective emission spectra.
The characteristic color sensitivity of the human eye has for its consequence that the blue light emission contributes least to the luminance (brightness) of a picture screen. In addition, the blue-emitting phosphors are not as efficient as the green- and red-emitting phosphors in plasma picture screens. These two effects have the result that the color temperature for white light is lower than desired in TV applications.
It is accordingly an object of the invention to provide a color picture screen provided with an improved blue-emitting phosphor layer.
This object is achieved by means of a color picture screen provided with a blue phosphor layer which comprises a first phosphor having a light emission in the range from 430 to 490 nm and a second phosphor having a light emission in the range from 380 to 450 nm.
Preferably, the second phosphor has a light emission in the range from 380 to 420 nm.
The combination of a first blue-emitting phosphor with a second phosphor whose light emission is still just in the visible range and is a deep violet achieves a blue light emission which appears xe2x80x9cbluerxe2x80x9d and has a strong color saturation. White light is obtained in this manner which has a high luminance and a high apparent color temperature.
It is particularly preferred that the second phosphor is chosen from the group comprising Tb3+-activated phosphors, Eu2+-activated phosphors, Bi3+-activated phosphors, Ga3+-activated phosphors, and Ce3+-activated phosphors.
It is particularly highly preferred that the second phosphor is chosen from the group comprising LaOBr:Tb, Y2O2S:Tb, Y3Al5O12:Tb, Ca3(PO4)2:Eu, Sr2P2O7:Eu, (Sr,Mg)2P2O7:Eu CaB2P2O9:Eu, CaSO4:Eu, CaO:Bi, ZnO:Ga and (Y,Gd)BO3:Ce.
All these phosphors efficiently emit light with a wavelength between 380 and 450 nm when excited by UV radiation or an electron ray.
It is preferred that the phosphor layer comprises a physical mixture of particles of the first phosphor and particles of the second phosphor.
It is favorable when the proportional quantity of the second phosphor in the phosphor layer lies between 5 and 50% by weight in relation to the quantity of the first phosphor.
This embodiment can be realized in a simple manner because the second phosphor may be simply added to the suspension of the phosphor with which the phosphor layer is manufactured.
It may also be preferred that the phosphor layer has a base layer comprising the first phosphor and a covering layer comprising the second phosphor.
It is preferred that the first phosphor is chosen from the group comprising ZnS:Ag, BaMgAl10O17:Eu and (Ba,Sr,Ca)5(PO4)3Cl:Eu.
The phosphor ZnS:Ag is particularly suitable for use as a blue-emitting phosphor in color cathode ray tubes because it emits blue light efficiently under excitation by an electron ray. The blue-emitting phosphors BaMgAl10I17:Eu and (Ba,Sr,Ca)5(PO4)3Cl:Eu are particularly suitable for use in plasma picture screens, because they combine a high color saturation with an efficient conversion of UV radiation into blue light. In addition, they are resistant to the thermal loads prevailing during the manufacture of the plasma picture screens.
It is furthermore preferred that the color picture screen is chosen from the group comprising cathode ray tubes, plasma picture screens, and liquid crystal picture screens.
These color picture screens provided with a blue phosphor layer according to the invention show an enhanced luminance, because higher proportional quantities of red and green light can be used for the color representation thanks to the improved blue light emission. The red and green light emissions contribute more strongly to the luminance of a color picture screen than does the blue light emission. It is furthermore advantageous that the color impression of white light is not changed in spite of the increased quantities of red and green light.